26 Papers from the Department of Marine Biology. 
poses, as Smith remarks, ‘‘a most peculiar gregarious habit in the 
larve,’’ and when we remember that marine larve are dependent for 
distribution, so far as we know, on chance vagaries of current rather 
than an internal migratory instinct, I for one can not suppose that so 
many larve should share a common path to their intended host and 
achieve such remarkable success in fixation. It is true that Delage, 
experimenting on the development of Sacculina, found that of a brood 
of Cypris larve, confined in a small aquarium in company with a crab, 
large numbers were found attached to the carapace of the crab; but 
this was certainly due to the stillness of the water in the aquarium and 
the small space in which the thousands of larve were confined, condi- 
tions which would hardly be repeated in nature. The fact that two 
Sacculine are occasionally, and even three rarely, found on a single host 
shows that contemporaneous infection does take place, but it also 
shows, I think, its comparative infrequency. 
I regret that I can not throw further light on the question of the con- 
tinuity of the root systems of adjacent parasites, for I think Geoffrey 
Smith’s statement requires confirmation. The time at my disposal 
was short and material came into my hands only early in my visit, so 
I did not preserve it carefully for histological study. Nor was I able 
either to make the search I had intended for the Peltogaster interna 
stage. But the whole life history is worthy of an exhaustive study by 
some worker on the Pacific coast having access to this rich source of 
material. 
THE EVOLUTION OF THOMPSONIA. 
If a budding process is actually proved to exist in Peltogaster and 
Peltogasterella it helps us to understand how such a form as Thompsonia 
has risen. When once the continuity of the root system in Thomp- 
sonia is demonstrated, it becomes impossible to regard it as a primitive 
representative of the group; it must, on the other hand, be admitted 
to be the most specialised of all. The root system is undoubtedly a 
new development unrepresented in the typical Cirripede; but in the 
course of endoparasitic development in Sacculina it appears before the 
central tumour, which in its complete development as external sac may 
still be said to show something of Cirripede structure. But the 
retardation of the appearance of the external sacs here foreshadowed 
reaches its climax in Thompsonia. There is here still further exalta- 
tion of the root system at their expense, for not only do the roots 
fulfil their nutritive functions, but they also produce germ cells. The 
root system is, in point of fact, the organism and the external sacs are 
merely externally placed ovaries, organs of a wholly transient nature. 
It is hardly possible to consider Thompsonia as a direct descendant 
of Peltogaster socialis, but the former genus may well have had an 
